1. Field of the Invention
The present invention relates to a set of resin sheets, and to a method for producing a ceramic structure using the set of resin sheets by laminating, integrating and firing a plurality of ceramic green sheets. Precisely, the invention relates to a ceramic structure having a surface recess or inner void in the surface thereof and a ceramic structure having an inner void thereinside, and to a method for producing the same.
2. Description of the Related Art
Heretofore, ceramic structures having a surface recess in a surface thereof have been applied to, for example, electronic parts-casing packages for casing semiconductor devices such as LSI or other various electronic parts therein, and demand for a ceramic structure having a complicated profile with an inner void therein is now increasing.
A ceramic structure of this type is such constituted, for example, when they are for electronic parts-casing packages, a metallized wiring layer is disposed on or inside an insulator that comprises laminated multiple insulating layers. Ceramics such as alumina ceramics are much used for the insulating layer, and recently, an insulator formed by laminating insulating layers has been put into practical use, in which the insulating layer is made of a sintered glass ceramic body that enables simultaneous firing with copper metallized wiring.
The ceramic structure of the type may be produced as follows: a suitable resin binder is added to a ceramic mix powder prepared by mixing the necessary components in a predetermined ratio, then the resulting mix powder is dispersed in an organic solvent to prepare a slurry, and this is formed into a ceramic green sheet (this may be hereinafter referred to as a green sheet) having a predetermined thickness according to a casting process such as a well-known doctor blade process or lip coater process.
Next, a metallized wiring layer is formed on the green sheet. Briefly, a suitable metal powder is mixed with a resin binder, a solvent and a solubilizer to prepare a metal paste, and this is printed on the green sheet to form a predetermined pattern therein according to a well-known screen-printing process. Through-holes are formed in the printed green sheet by the use of a microdrill or a laser, and the through-holes are filled with the metal paste to be through-conductors (via-conductors, via-holes).
The subsequent step is described for two cases, (1) one is a ceramic structure having a surface recess in the surface thereof; and (2) the other is a ceramic structure having an inner void thereinside.
In case (1), a through-hole is punched in a predetermined site of a green sheet for the purpose of forming a surface recess in the surface thereof. Next, as in FIG. 3A of FIGS. 3A and 3B showing conventional processes, a plurality of green sheets 21a and 21b each with the through-hole 20 formed therein are laminated with other green sheets 21c, 21d and 21e by the use of a suitable adhesive liquid, and the resulting green sheet laminate having a surface recess 22 formed in its surface is fired under a predetermined condition to obtain a ceramic structure 23 of FIG. 3B.
In case (2), a green sheet is laminated on a ceramic green sheet with the through-hole constituting the inner void by the use of a suitable adhesive liquid, and the resulting green sheet laminate is fired under a predetermined condition to give a ceramic structure having a void thereinside.
These days ceramic structures of the type are desired to be downsized with the recent tendency in the art toward multi-function and high-function devices, and it is indispensable to prevent the green sheets therefor from being deformed when laminated. Further, it is also desired to provide more complicated ceramic structures having multiplesurface recesses or inner voids. As a related art, there are Japanese Unexamined Patent Publication JP-A 2003-318541 and Japanese Unexamined Patent Publication JP-A 2003-332741.
However, when a green sheet with a through-hole to form a surface recess or inner void is laminated under pressure with a green sheet with no through-hole to produce a ceramic structure having a surface recess or inner void, there occurs a pressure difference between the surface recess or inner void part and the other part and, as a result, the bottom of the surface recess or inner void of the green sheet laminate not receiving pressure may swell. When an electronic part is fitted to the bottom of the swollen surface recess or inner void, then there may occur bonding failures.
When the pressure to be given for the bonding is lowered so as to solve the problem, then the pressure necessary for the bonding of the peripheral parts around the surface recess or inner void lowers and, as a result, the green sheets may delaminate. In that condition, it is difficult to obtain a ceramic structure of high reliability with no structure defects.
On the other hand, when the wall that surrounds the surface recess or inner void of a green sheet is thin and weak, then there may occur a problem in that the thin wall part will be deformed under pressure in lamination; or when multiple surface recesses or inner voids spaced from each other with a narrow distance therebetween are formed in one green sheet by punching the green sheet or when one green sheet with multiple surface recesses or inner voids formed therein is snapped into individual pieces for ceramic structures by the use of a cutting edge of metal or the like in a process of green sheet working, then there may also occur a problem in that the thin wall part around the surface recesses or inner voids will be deformed.
For solving the problems, a method may be taken into consideration, which comprises filling a resin sheet capable of being removed through thermal decomposition in firing, in the site to form a surface recess or inner void in a green sheet laminate and laminating the thus-filled green sheets.
However, at the step of thermal decomposition of the resin sheet in firing, the melted substance of the resin sheet dissolves the resin binder in the green sheet that is in contact with the resin sheet, and therefore there occurs a problem in that the green sheet is thereby deformed.
In addition, when the resin binder in a green sheet that is in contact with a resin sheet is thermally decomposed (removed) and while the amount of the resin binder in the green sheet decreases, then the strength of the green sheet lowers. In such a case, when the resin sheet is thereafter thermally decomposed on the brittle green sheet, then the resin sheet melt of high viscosity that has become liquid under heat is further thermally decomposed on the surface of the green sheet that has become brittle after resin sheet removal, while boiling, vibrating and moving on every side. As a result, there occurs a still another problem in that the surface of the green sheet that is in contact with the thermally decomposed part is partly hollowed out and the surface thereof is therefore eroded and broken.
These problems are especially remarkable when the amount of the resin sheet to be fitted to a green sheet is large. Specifically, at the step of binder removal, the entire resin sheet is simultaneously thermally decomposed and therefore a large amount of a melted matter is formed all at a time. In addition, the vapor resulting from the vaporization of the melted matter is formed also all at a time, and it may be believed that the damage of the green sheet that is in contact with the resin sheet is will be great.